Optical cavity including a light emitting device and wavelength converting material

ABSTRACT

Light emitting devices are described herein. A light-emitting device includes a substrate having a surface below an optical cavity, one or more light emitting diodes (LEDs) disposed above the surface of the substrate, a first wavelength-converting layer, and a second wavelength-converting layer. The first wavelength-converting layer is disposed on the surface of the substrate below the optical cavity, covers the entire surface of the substrate except for portions of the surface of the substrate that are situated underneath any of the one or more LEDs, and has a thickness that is equal to or less than a thickness of at least one of the one or more LEDs. The second wavelength-converting layer is disposed above the optical cavity.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/388,877, filed Sep. 29, 2014, which is the U.S. National Phaseapplication under 35 U.S.C. § 371 of International Application No.PCT/IB2013/052572, filed on Mar. 30, 2013, which claims the benefit ofU.S. Patent Application No. 61/617,919, filed on Mar. 30, 2012. Theseapplications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an optical cavity including asemiconductor light emitting device such as light emitting diode and awavelength converting material such as a phosphor.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes such as surface-emitting lasers (VCSELs), and edge emittinglasers are among the most efficient light sources currently available.Materials systems currently of interest in the manufacture ofhigh-brightness light emitting devices capable of operation across thevisible spectrum include Group III V semiconductors, particularlybinary, ternary, and quaternary alloys of gallium, aluminum, indium, andnitrogen, also referred to as III nitride materials. Typically, IIInitride light emitting devices are fabricated by epitaxially growing astack of semiconductor layers of different compositions and dopantconcentrations on a sapphire, silicon carbide, III-nitride, or othersuitable substrate by metal-organic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), or other epitaxial techniques. The stackoften includes one or more n-type layers doped with, for example, Si,formed over the substrate, one or more light emitting layers in anactive region formed over the n-type layer or layers, and one or morep-type layers doped with, for example, Mg, formed over the activeregion. Electrical contacts are formed on the n- and p-type regions.

III-nitride devices may be combined with wavelength converting materialssuch as phosphors, as is known in the art, to form white light or lightof other colors.

FIG. 1 illustrates a backlight described in more detail in U.S. Pat. No.7,052,152. Column 5 line 33 through column 6 line 7 of U.S. Pat. No.7,052,152 describes the device in FIG. 1 as a backlight configurationwhen only blue, UV, or near-UV LEDs are used, and where thecolor-converting phosphor layer 139 is on the cover plate 140. The coverplate 140 may or may not be a diffuser, depending on the amount ofdiffusion performed by the phosphor. The phosphor layer 139 is a uniformlayer, consisting of one or more different type of phosphors.Preferably, a green and a red phosphor are used, but a yellow (YAG)phosphor could be used as well. This layer 139 can, for example, beapplied by spray painting, screen-printing, or electrophoreticdeposition, or might be a film with uniform density of particles or aluminescent dye distributed throughout the film. This configuration isattractive because the phosphor is not on top of the LED die, and lightemitted from the phosphor to the rear of the backlight 126 has a largerrecycling efficiency than into the LED chips, due to the highreflectivity of the films used in the backlight 126. And in addition tothe recycling efficiency, the phosphor can be operated at a lowertemperature and does not have chemical compatibility issues with the LEDdie, improving the efficiency and lifetime considerably. An LCD panel114 is disposed over backlight 126.

In another embodiment, one type of phosphor is applied to the coverplate 140, preferably the green or amber phosphor, while anotherphosphor, preferably the red phosphor, is applied to the rear panel 148of the backlight configuration. The rear panel acts as a diffuser. Thisphosphor is not applied as a uniform coating, but is applied as a dotpattern. The combination of blue light from the LEDs and the red andgreen light from the phosphor layers produces a substantially whitebacklight for the LCD panel 114. By separating the phosphor in such aconfiguration, higher conversion efficiency is achieved, while byoptimizing the size and spacing of the phosphor dots the required colorbalance and gamut can be achieved.

SUMMARY

It is an object of the invention to form an optical system where an LEDand a wavelength converting material are disposed on a substrate, whichmay improve the efficiency of the system.

Embodiments of the invention include a semiconductor light emittingdiode attached to a substrate. A first region of wavelength convertingmaterial is disposed on the substrate. The wavelength convertingmaterial is configured to absorb light emitted by the semiconductorlight emitting diode and emit light at a different wavelength. In thefirst region, the wavelength converting material coats an entire surfaceof the substrate. The substrate is disposed proximate a bottom surfaceof an optical cavity. A second region of wavelength converting materialis disposed proximate a top surface of the optical cavity.

Embodiments of the invention include a semiconductor light emittingdiode attached to a substrate. A first region of wavelength convertingmaterial is disposed on the substrate. The wavelength convertingmaterial is configured to absorb light emitted by the semiconductorlight emitting diode and emit light at a different wavelength. Thesubstrate is disposed proximate a bottom surface of an optical cavity. Asecond region of wavelength converting material is disposed proximate atop surface of the optical cavity. A reflective surface is disposedabove the semiconductor light emitting diode. The reflective surface isconfigured to direct reflected light away from the semiconductor lightemitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art backlight including a two dimensionalarray of LEDs.

FIG. 2 illustrates an optical cavity with at least one semiconductorlight emitting device attached to a substrate and a wavelengthconverting material disposed on the substrate.

FIG. 3 illustrates an optical cavity with at least one semiconductorlight emitting device attached to a substrate and wavelength convertingmaterial disposed at the top and bottom of the cavity.

FIG. 4 illustrates an optical cavity with at least one semiconductorlight emitting device attached to a substrate, wavelength convertingmaterial disposed at the sides of the cavity, and reflective regionsdisposed over the semiconductor light emitting devices.

FIGS. 5, 6, and 7 illustrate optical cavities with shaped reflectiveregions.

DETAILED DESCRIPTION

FIG. 2 illustrates an optical cavity according to embodiments of theinvention. One or more III-nitride LEDs 16 are disposed on a substrate14 in or next to optical cavity 10. Though the examples below refer toIII-nitride LEDs that emit blue or UV light, semiconductor lightemitting devices besides LEDs such as laser diodes and semiconductorlight emitting devices made from other materials systems such as otherIII-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, orSi-based materials may be used in embodiments of the invention.

Any suitable III-nitride LED may be used and such LEDs are well known.LEDs 16 may be, for example, flip chip devices configured to emit amajority of light from the top surface of the LED. To form such an LED,a III-nitride semiconductor structure is first grown on a growthsubstrate, as is known in the art. The growth substrate may be anysuitable substrate such as, for example, sapphire, SiC, Si, GaN, orcomposite substrates. The semiconductor structure includes a lightemitting or active region sandwiched between n- and p-type regions. Ann-type region may be grown first and may include multiple layers ofdifferent compositions and dopant concentration including, for example,preparation layers such as buffer layers or nucleation layers, and/orlayers designed to facilitate removal of the growth substrate, which maybe n-type or not intentionally doped, and n- or even p-type devicelayers designed for particular optical, material, or electricalproperties desirable for the light emitting region to efficiently emitlight. A light emitting or active region is grown over the n-typeregion. Examples of suitable light emitting regions include a singlethick or thin light emitting layer, or a multiple quantum well lightemitting region including multiple thin or thick light emitting layersseparated by barrier layers. A p-type region may then be grown over thelight emitting region. Like the n-type region, the p-type region mayinclude multiple layers of different composition, thickness, and dopantconcentration, including layers that are not intentionally doped, orn-type layers. The total thickness of all the semiconductor material inthe device is less than 10 μm in some embodiments and less than 6 μm insome embodiments.

A metal p-contact is formed on the p-type region. If a majority of lightis directed out of the semiconductor structure through a surfaceopposite the p-contact, such as in a flip chip device, the p-contact maybe reflective. A flip chip device may be formed by patterning thesemiconductor structure by standard photolithographic operations andetching the semiconductor structure to remove a portion of the entirethickness of the p-type region and a portion of the entire thickness ofthe light emitting region, to form a mesa which reveals a surface of then-type region on which a metal n-contact is formed. The mesa and p- andn-contacts may be formed in any suitable manner. Forming the mesa and p-and n-contacts is well known to a person of skill in the art.

The semiconductor structure may be connected to a support through the p-and n-contacts. The support is a structure that mechanically supportsthe semiconductor structure. The support is a self-supporting structuresuitable to attach to substrate 14 of FIG. 2. For example, the supportmay be reflow-solderable. Any suitable support may be used. Examples ofsuitable supports include an insulating or semi-insulating wafer withconductive vias for forming electrical connections to the semiconductorstructure, such as a silicon wafer, thick metal bonding pads formed onthe semiconductor structure, for example by plating, or a ceramic,metal, or any other suitable mount.

LEDs 16, which include the semiconductor structure, metal contacts, andsupport described above, are mounted on substrate 14. Substrate 14 maybe optically reflective and thermally conductive. Electrical contact toLEDs 16 may be made through substrate 14. Examples of suitablesubstrates 14 include metal core printed circuit board, FR4-basedprinted circuit board, ceramic, metal, plastic, and silicone. In someembodiments, the top surface of substrate 14 is reflective, or is coatedwith a reflective substance such as a reflective paint or a layer of areflective metal. Substrate 14 may have a thermal conductivity of atleast 0.1 W/mK (silicone) in some embodiments, at least 10 W/mK in someembodiments, and at least 100 W/mK in some embodiments, and between 0.1W/mK and 400 W/mK (copper) in some embodiments.

Though multiple LEDs are illustrated in each of FIGS. 2, 3, and 4, insome embodiments of the invention a single LED 16 may be used. MultipleLEDs 16 may be arranged in any suitable arrangement on substrate 14. Forexample, four LEDs 16 may be arranged in a two by two square array onsubstrate 14, or multiple LEDs 16 may be arranged in a circle onsubstrate 14. Many variations are possible and within the scope of theinvention.

Next to LEDs 16, one or more wavelength converting regions 18 aredisposed on substrate 14. In devices including multiple LEDs 16,wavelength converting regions 18 may be disposed between LEDs 16. Insome embodiments, rather than being formed as dots that only partiallycover the surface of the substrate in the region where the wavelengthconverting region 18 is formed as in the device illustrated in FIG. 1,wavelength converting region 18 covers the entire surface of thesubstrate in the region where it is formed.

In some embodiments, there is no gap between the LEDs 16 and the edge ofwavelength converting regions 18. For example, LEDs 16 may be mounted onsubstrate 14, then the entire structure including the LEDs 16 andsubstrate 14 is coated with wavelength converting material such thatthere is no gap between wavelength converting regions 18 and LEDs 16.

In some embodiments, there is a small gap between the LEDs 16 and theedge of wavelength converting regions 18. For example, wavelengthconverting regions 18 may be formed on substrate 14, then LEDs 16 aremounted in the areas between wavelength converting regions.Manufacturing tolerances require that a small gap between the edge of anLED 16 and the edge of a wavelength converting region 18 will not becovered with wavelength converting material.

Wavelength converting regions 18 are at least 0.5 mm wide in someembodiments and no more than 20 mm wide in some embodiments. Any gapbetween an LED 16 and the edge of a wavelength converting region 18 isbetween 0 mm and 0.5 mm in some embodiments. Wavelength convertingregions 18 may be the same thickness as LEDs 16 in some embodiments. Inthe alternative, wavelength converting regions 18 may be thinner orthicker than LEDs 16.

Wavelength converting material in wavelength converting regions 18absorbs light emitted by LEDs 16 and emits light of a differentwavelength. Unconverted light emitted by the light emitting device isoften part of the final spectrum of light extracted from the opticalcavity, though it need not be. Examples of common combinations include ablue-emitting LED combined with a yellow-emitting wavelength convertingmaterial, a blue-emitting LED combined with green- and red-emittingwavelength converting materials, a UV-emitting LED combined with blue-and yellow-emitting wavelength converting materials, and a UV-emittingLED combined with blue-, green-, and red-emitting wavelength convertingmaterials. Wavelength converting materials emitting other colors oflight may be added to tailor the spectrum of light emitted from thedevice.

The wavelength converting material may be conventional phosphors,organic phosphors, quantum dots, organic semiconductors, II-VI or III-Vsemiconductors, II-VI or III-V semiconductor quantum dots ornanocrystals, dyes, polymers, or materials such as GaN that luminesce.Any suitable phosphor may be used, including but not limited togarnet-based phosphors such as Y₃Al₅O₁₂:Ce (YAG), Lu₃Al₅O₁₂:Ce (LuAG),Y₃Al_(5-x)Ga_(x)O₁₂:Ce (YAlGaG), (Ba_(1-x)Sr_(x))SiO₃:Eu (BOSE), andnitride-based phosphors such as (Ca,Sr)AlSiN₃:Eu and(Ca,Sr,Ba)₂Si₅N₈:Eu.

Any of the wavelength converting regions described herein may include awavelength converting material disposed in a matrix of transparentmaterial, such as silicone, epoxy, glass, or any other suitablematerial. The wavelength converting regions described herein may be, forexample, thin films, ceramic slabs pre-formed or formed in situ with orwithout binder material other than the wavelength converting materialitself, particles or powdered wavelength converting material mixed witha transparent binder such as silicone, epoxy, or any other suitablematerial and formed by screen printing, molding, spray coating,stenciling, or any other suitable technique. Each wavelength convertingregion described herein may include a single wavelength convertingmaterial, a mixture of wavelength converting materials, or multiplewavelength converting materials formed as separate layers rather thanmixed together. Wavelength converting materials emitting differentcolors of light may be disposed in separate regions or mixed together.

The thickness of wavelength converting regions 18 depends on thewavelength converting materials and the deposition technique. Thethickness of the wavelength converting regions may be between at least0.5 μm in some embodiments, no more 500 μm in some embodiments, and nomore than 1 mm in some embodiments. In various embodiments, wavelengthconverting material may be disposed over the tops of LEDs 16, or notdisposed over the tops of LEDs 16, as illustrated in FIGS. 2, 3, and 4.

In some embodiments, the thickness and density of the wavelengthconverting material in wavelength converting regions 18 are selectedsuch that little or no light incident on the top of wavelengthconverting regions 18 penetrates wavelength converting regions 18 toreach substrate 14. A wavelength converting layer formed such that nolight reaches substrates may be advantageous for wavelength convertingmaterials that are highly scattering and/or in applications where it isdesirable to wavelength-convert a large fraction of light emitted by theLEDs 16. For example, in some embodiments where the structure emits warmwhite light, at least 90% of the light emitted by LEDs 16 iswavelength-converted by the wavelength converting regions.

In alternative embodiments, the thickness and density of wavelengthconverting material in wavelength converting regions 18 are selectedsuch that at least some light is transmitted through the wavelengthconverting material and reflects off the substrate back through thewavelength converting material. Such a thinner layer may be advantageousfor wavelength converting materials that are not highly scattering, suchas, for example, organic phosphors, quantum dots, or dense ceramics; inapplications where it is desirable to wavelength-convert a smallerfraction of light emitted by the LEDs 16; and/or when the wavelengthconverting material is expensive.

The substrate 14, LEDs 16, and wavelength converting regions 18 form thebottom surface of an optical cavity 10. Cavity 10 when viewed from thetop may be any suitable shape, such as square or round. The sidewalls ofcavity 10 need not be vertical as illustrated in FIGS. 2, 3, and 4, theymay be sloped to form, for example, a truncated inverted pyramid orcone. The shape of cavity 10 may be dictated by the application. Cavity10 may be a box filled with air (index of refraction of 1) or ambientgas, or a solid transparent material such as glass (index of refractionof 1.4-2.2), silicone (index of refraction of 1.4-2), transparentceramic (index of refraction of 1.8), sapphire (index of refraction of1.76), SiC (index of refraction of 2.4), cubic zirconia (index ofrefraction of 2.15), diamond (index of refraction of 2.4), GaN (index ofrefraction of 2.4), epoxy, or any other suitable material. In someembodiments, a solid material for cavity 10 is selected to have arefractive index as close as possible to the refractive index of LEDs 16(III-nitride material with an index of refraction that varies but isoften about 2.4). Optical cavity 10 has an index of refraction of atleast 1 in some embodiments, no more than 2.4 in some embodiments, atleast 1.8 in some embodiments, and at least 2 in some embodiments. Thethickness of a solid, transparent material forming optical cavity 10 maybe at least 0.5 mm in some embodiments and no more than 3 mm in someembodiments, though both thicker and thinner optical cavities 10 arepossible and within the scope of the invention.

In the case of an optical cavity 10 that is a solid transparentmaterial, the structure including substrate 14, LEDs 16, and wavelengthconverting regions 18 may be attached to the solid transparent materialby an adhesive such as silicone, or by another other suitable techniqueor material.

LEDs 16 and the wavelength converting regions 18 are both disposed on ornear substrate 14, where heat may be efficiently dissipated throughsubstrate 14, for example by configuring substrate 14 as a heat sink orby thermally connecting substrate 14 to a heat sink. Because wavelengthconverting regions 18 can be kept relatively cool during operation overa variety of driving conditions, problems associated with heating, suchas reduced quantum efficiency and shifting of the emitted wavelength,are reduced or eliminated. In addition, because wavelength convertingregions 18 not formed directly in the path of light from LEDs 16, theincident optical intensity on the wavelength converting regions is keptlow, which may improve the quantum efficiency of the wavelengthconverting material. The thermal resistivity of the path between theLEDs 16 and the substrate 14, or of the path between the wavelengthconverting regions 18 and substrate 14, is at least 0.1 cm²K/W in someembodiments, no more than 50 cm²K/W in some embodiments, at least 1cm²K/W in some embodiments, and no more than 2 cm²K/W in someembodiments.

In some embodiments, the sidewalls 12 of optical cavity 10 arereflective. In the case where cavity 10 is a box, sidewalls 12 may bereflective, solid walls, or a reflective material disposed on orattached to solid walls. In the case where cavity 10 is a solidtransparent material, sidewalls 12 may be a reflective foil orreflective material disposed on the sides of the solid transparentmaterial.

In some embodiments, an optional filter 20 is disposed on the top ofoptical cavity 10. For example, filter 20 may be an interference-baseddichroic reflecting layer, or any other suitable filter. In the casewhere cavity 10 is a box, a dichroic filter 20 may be a sheet ofdichroic material, or a dichroic layer coated or formed on the top orbottom of a transparent sheet such as glass or plastic, which may or maynot function as a cover over the structure illustrated in FIG. 2. In thecase where cavity 10 is a solid transparent material, a dichroic filter20 may be coated or formed on the top surface of the solid transparentmaterial.

Filter 20 is configured to reflect all or a majority of light emitted byLEDs 16 and incident on filter 20 at small angles relative to a normalto the top surface of LEDs 16, as illustrated by ray 24. Such light isreflected, as illustrated by ray 28. In some embodiments, filter 20 isoptionally configured to be less reflective of light emitted by LEDs 16and incident on filter 20 at large angles relative to a normal to thetop surface of LEDs 16, such that some large-angle light escapes, asillustrated by ray 26. Filter 20 may also be configured to transmit allor a majority of light that is wavelength converted by wavelengthconverting regions 18 at any angle, as illustrated by ray 30.

An optional scattering layer 22 may be formed over filter 20, or overoptical cavity 10 in embodiments that do not include a filter 20.Scattering layer 22 may include scattering particles such as TiO₂ or anyother suitable material disposed in a transparent matrix such assilicone, epoxy, glass, or any other suitable material. In someembodiments, instead of or in addition to scattering layer 22,scattering particles may be disposed within the total volume or part ofthe volume of cavity 10.

During operation, a fraction of light emitted from LEDs 16 is redirectedby optional filter 20 and optional scattering layer 22 back toward thebottom surface of cavity 10. Light incident on wavelength convertingregions 18 is converted, i.e. absorbed and reemitted at a longerwavelength. Converted light is directed upward by scattering fromwavelength converting regions 18 and by reflective substrate 14 intocavity 10. The converted light and a fraction of unconverted light fromLEDs 16 escapes through filter 20, possibly after one or morereflections off the reflective edges of cavity 10, and/or possibly afterbeing scattered by scattering layer 22 or other surfaces. Theabove-described reflection and scattering events serve to mix convertedand unconverted light, and distribute unconverted light more uniformlyover the wavelength converting regions 18, which may reduce “hot spots”,i.e. spots on wavelength converting regions 18 that experience higherrelative intensity of unconverted light. Since many suitable wavelengthconverting materials are most efficient when the intensity ofunconverted pump light is low, the more uniform excitation helps improveoverall system efficiency.

The fraction of unconverted light from LEDs 16 that is transmittedthrough filter 20 may be adjusted to ensure that the desired fraction ofthe pump light escapes the cavity. For example, for a white colorpoint,in various embodiments 5% to 30% of the light escaping cavity 10 shouldbe unconverted light from LEDs 16, depending on the desired colortemperature of the mixed light, and depending on the spectral propertiesof the light emitted from LEDs 16 and wavelength converting regions 18.The fraction of unconverted light that is transmitted may be adjusted inembodiments including a dichroic filter 20 by selecting the materialsused in the layers of the dichroic filter, the thicknesses of the layersand the number of layers in the dichroic filter, as is known in the art.In embodiments including a scattering layer 22, the fraction ofunconverted light that is transmitted may be adjusted by increasing ordecreasing scattering to increase or decrease conversion. Scattering canbe increased by increasing the concentration of scattering particles,increasing the thickness of the scattering layer, changing thecomposition of the scattering particles, changing the size and shape ofthe scattering particles or changing the composition of the matrix inwhich the scattering particles are embedded. Scattering can be also beincreased by roughening surfaces of optical cavity 10, such as the topor side surfaces of optical cavity 10. In some embodiments, the fractionof unconverted light is increased by decreasing the thickness and/orconcentration of wavelength converting material in the wavelengthconverting regions. In some embodiments, the fraction of unconvertedlight is increased by introducing scattering particles into wavelengthconverting region 18, which causes a larger fraction of incident lightto be scattered away from the wavelength converting material, ratherthan absorbed by the wavelength converting material. Scatteringparticles can be disposed in a separate layer above the wavelengthconverting material, or mixed with the wavelength converting material.In some embodiments, the fraction of unconverted light that istransmitted is increased by forming scattering layer 22 and/or filter 20on only a portion of the top of optical cavity 10.

In some embodiments, filter 20 and scattering layer 22 are adjusted toreduce angular variations in the color of light emitted from thestructures illustrated in FIGS. 2, 3, and 4. In some embodiments,angular variations are reduced by increasing the scattering at the topof optical cavity 10, for example by adding scattering particles at thetop surface, adding scattering wavelength converting particles at thetop surface, roughening the top surface of cavity 10, and/or addingscattering particles to the bulk of cavity 10. In some embodiments,angular variations are reduced by adjusting the angular transmittanceproperties of filter 20 (if present) to selectively transmit light atdesirable angles, as is known in the art. In some embodiments, angularvariations are reduced by adjusting the amount of wavelength convertingmaterial added to the sidewalls of optical cavity 10 and/or by adjustingtransmission of light through the sidewalls.

FIG. 3 illustrates an optical cavity 10 with wavelength convertingregions disposed at the top and bottom of the optical cavity. In thestructure of FIG. 3, first wavelength converting regions 18 are disposedbetween LEDs 16 as in FIG. 2. Second wavelength converting regions 36are disposed at the top of cavity 10, for example just below or justabove filter 20 in embodiments including filter 20, or just below orjust above scattering layer 22 in embodiments including scattering layer22. Second wavelength converting region 36 may be a single, continuousregion that covers the entire top of cavity 10, as illustrated in FIG.3, or it may be formed in smaller regions separated by spaces,transparent material, or another material such as a scattering material.Second wavelength converting region 36 may be formed by any suitabletechnique on the top surface of cavity 10, the bottom surface of filter20, the top surface of filter 20, or on a separate sheet of transparentmaterial, for example.

Any of the above-described characteristics of wavelength convertingregions 18, including but not limited to materials, thickness, anddeposition techniques, may be applied to wavelength converting regions36 in embodiments of the invention. Wavelength converting regions 18 and36 need not have the same characteristics, though they may in someembodiments.

In some embodiments, a second wavelength converting material is mixedwith the scattering particles in scattering layer 22, rather than formedas a second, separate layer 36 as illustrated in FIG. 3.

A single or multiple wavelength converting materials may be included ineach of wavelength converting regions 18 and wavelength convertingregion 36. Wavelength converting materials emitting different colors oflight may be separated or mixed. Scattering or othernon-wavelength-converting materials may be added to either or both ofwavelength converting regions 18 and 36.

In some embodiments, in systems with wavelength converting regions onthe top and bottom of cavity 10, the reflectivity of the top surface ofcavity 10 (i.e. the reflectivity of filter 20) can be reduced to allowmore light to be transmitted by filter 20, which may improve theefficiency of the structure. Since there is a top wavelength convertingregion, less light from LEDs 16 is required to be incident on the bottomwavelength converting regions, therefore less light must be reflectedtoward the bottom of optical cavity 10. As a result, filter 20 may beless reflective. In addition, scattering caused by the wavelengthconverting region on the top of the cavity 10 may increase the amount oflight directed toward wavelength converting regions 18 on the bottom ofthe cavity, and may reduce the amount of scattering material needed inscattering layer 22 or may eliminate the need for scattering layer 22altogether.

In some embodiments, the wavelength converting regions disposed on thebottom of cavity 10 include the wavelength converting materials thatsuffer the worst degradation of performance with increased temperature,since the wavelength converting regions on the bottom of cavity 10 canbe more readily cooled than the wavelength converting regions on the topof cavity 10.

In some embodiments, the device includes wavelength converting materialA which can be excited by light emitted by another wavelength convertingmaterial B. Wavelength converting material A may be disposed in thewavelength converting regions on the bottom of cavity 10, whilewavelength converting material B may be disposed in the wavelengthconverting region on the top of cavity 10, to minimize interactionbetween the wavelength converting materials.

For example, devices that make white light often include a blue LED andred- and green- or yellow-emitting phosphors. Many phosphors that emitred light will absorb light emitted by the green- or yellow-emittingphosphor. Many red-emitting phosphors are also temperature sensitive. Insome embodiments, a red-emitting phosphor is disposed on the bottom ofcavity 10 and a green- or yellow-emitting phosphor is disposed on thetop of cavity 10. In such an arrangement, the red-emitting phosphor mayabsorb less green or yellow light than in a system where the red- andgreen/yellow-phosphors are mixed, which may improve the color renderingindex of the mixed light, may improve the efficiency of the device, andmay simplify color targeting.

In some embodiments, a red-emitting phosphor is disposed on the top ofcavity 10 and a green- or yellow-emitting phosphor is disposed on thebottom of cavity 10. In some embodiments, a red-emitting andgreen/yellow-emitting phosphor are mixed, and the mixture is disposed onboth the top and bottom of cavity 10. In some embodiments, a mixture ofa red-emitting phosphor and red-emitting quantum dots is disposed on thebottom of cavity 10 and a mixture of multiple types of green-emittingphosphors is disposed on the top of cavity 10. In some embodiments, asingle red-emitting phosphor is disposed on the bottom of cavity 10 anda mixture of the same or a different red-emitting phosphor and agreen/yellow-emitting phosphor is disposed on the top of cavity 10.

During operation of the system illustrated in FIG. 3, a fraction of thelight 26 from LEDs 16 is transmitted unconverted through the top of thecavity 10 and is extracted. Unconverted light may also reflect severaltimes inside the cavity 10 before escaping cavity 10. Another fractionof the light from the LEDs 16 is absorbed by wavelength convertingregion 36 and reemitted at a different wavelength. Some of thisreemitted light escapes immediately and some enters the cavity 10 whereit may reflect a number of times before being extracted out the top ofcavity 10. A final portion 24 of the light from LEDs 16 reflects off ofthe top of the cavity 10 and is incident on wavelength convertingregions 18. Wavelength converting regions 18 absorb some of this lightand reemit it at a different wavelength, and reflect the rest.Wavelength converted or reflected unconverted light may reflect a numberof times before being extracted out of the top of cavity 10.

FIG. 4 illustrates possible modifications to either of the structuresillustrated in FIGS. 2 and 3. In some embodiments, the area of the topof cavity 10 directly over LEDs 16 is made more reflective than filter20 in embodiments where a filter 20 is included, or is made reflectivein embodiments where a filter 20 is not included. For example,reflective regions 38 may be formed directly over LEDs 16 at the top ofcavity 10, such that light from LEDs 16 encounters reflective regions 38before filter 20 or scattering layer 22. Reflective regions 38 may be,for example, disks of reflective metal foil or other reflective materialdisposed on the top of cavity 10 or on the bottom of filter 20, on atransparent cover, or on any other suitable layer.

If the light reflected by reflective regions 38 is directed back towardsLEDs 16, the light may be absorbed by LEDs 16 and lost. In someembodiments, reflective regions 38 are configured to direct reflectedlight away from LEDs 16. For example, the reflective regions illustratedin FIG. 4 may have a roughened, textured, or patterned surface thatdirects the reflected light away from LEDs 16. FIGS. 5, 6, and 7illustrate examples of shaped reflective regions 38 that directreflected light away from LEDs 16. Though FIGS. 5, 6, and 7 illustrateone LED 16, the reflective regions 38 illustrated can be repeated overmultiple LEDs in a cavity 10. The reflective regions 38 illustrated areoften rotationally symmetric around a normal to the top surface of LED16, though they need not be in some embodiments.

The reflective region 38 illustrated in FIG. 5 has a surface 41 thatrises from proximate the center of LED 16 toward the top of cavity 10,then extends back toward wavelength converting regions 18. As thesurface 41 extends down toward wavelength converting regions 18, it getsfurther from LED 16. The surface 41 is illustrated in FIG. 5 ascontacting both the top surface of LED 16 and the top surface ofwavelength converting region 18, though it need not contact either orboth surfaces in some embodiments. As illustrated by ray 43, lightemitted from LED 16 is reflected off surface 41 away from LED 16 andtoward wavelength converting region 18. The surface 41 may be curved asillustrated in FIG. 5 or flat, and it may include both curved portionsand flat portions. In some embodiments, top wavelength converting region36 may be omitted and wavelength converting material may be disposed inregion 42.

FIG. 7 illustrates a system including a reflective region 38 with thesame shape as in FIG. 5. In FIG. 7, region 42 of FIG. 5 is omitted andthe surface 41 of reflective region 38 forms the top surface of opticalcavity 10. Wavelength converting region 36 is formed over reflectiveregion 38 as a conformal coating, i.e. a coating with a substantiallyuniform thickness. For example, the thickness of wavelength convertingregion 36 in FIG. 7 may vary by less than 50%. The structure illustratedin FIG. 7 may be formed by molding optical cavity 10 over substrate 14,LED 16, and wavelength converting region 18, then molding wavelengthconverting region 36 over optical cavity 10, or pressing a preformed,flexible wavelength converting sheet over optical cavity 10. As in FIG.5, the reflective region 38 need not have the precise shape illustratedin FIG. 7.

The reflective region 38 illustrated in FIG. 6 has a surface 47 thatrises diagonally from a region of optical cavity 10 above and proximatethe center of LED 16 up toward wavelength converting region 36 and awayfrom LED 16. Surface 47 may also be considered a protrusion such as aninverted cone or pyramid that extends from a base at the top of cavity10 toward a point positioned below the top of cavity 10 and above LED16. The surface 47 is illustrated in FIG. 6 as being spaced apart fromthe top surface of LED 16, though it may contact LED 16 in someembodiments. As illustrated by ray 45, light emitted from LED 16 isreflected off surface 47 away from LED 16 and toward wavelengthconverting region 18. The surface 47 may be flat as illustrated in FIG.6 or curved, and it may include both curved portions and flat portions.

In some embodiments, the structures illustrated in FIGS. 5 and 6 reflectby total internal reflection, by creating a difference in index ofrefraction at surfaces 41 and 47 between the top of reflective region 38(42 in FIG. 5, 44 in FIG. 6) and the bottom of reflective region 38(cavity 10). Because the reflective regions 38 reflect by total internalreflection, light incident on the surfaces 41 and 47 at angles less thanthe critical angle is transmitted through the surfaces rather thanreflected. The structures may be molded in some embodiments. Forexample, the structure of FIG. 5 may be molded on wavelength convertingregion 36 or another suitable structure such as a transparent cover,then positioned over substrate 14 such that cavity 10 is air and region42 is molding compound, silicone, or any other suitable material. Thestructure of FIG. 6 may be molded over substrate 14 such that region 44is air and cavity 10 is molding compound, silicone, or any othersuitable material. In some embodiments, the difference in index ofrefraction that forms reflective regions 38 is between air and glass orany of the materials described above as suitable to form cavity 10. Forexample, regions 42 and 44 may be glass and cavity 10 may be air, orregions 42 and 44 may be air and cavity 10 may be glass. The differencein index of refraction between the two materials forming the surfacesillustrated in FIGS. 5 and 6 may be at least 0.4 in some embodiments, atleast 0.5 in some embodiments, at least 0.8 in some embodiments, and atleast 1 in some embodiments. In some embodiments, the reflective regions38 illustrated in FIGS. 5 and 6 are formed or coated with reflectivematerial.

Returning to FIG. 4, in some embodiments, wavelength converting and/orscattering materials are disposed on the sides of optical cavity 10. Forexample, wavelength converting regions 40 may be formed on the sidewallsof cavity 10. Wavelength converting regions may include a wavelengthconverting material such as a phosphor that is mixed with a transparentmaterial such as silicone, then coated on the sides of cavity 10,disposed on a transparent sheet of material that is positioned adjacentthe sides of cavity 10, or formed into a film that is attached to thesides of cavity 10. Light encountering wavelength converting regions 40is absorbed and reemitted at a different wavelength or reflected.

Embodiments of the invention may have advantages. Since at least part ofthe wavelength converting material is disposed such that it can be keptcool, such as on substrate 14, efficiency of the system may be improvedover systems where the wavelength converting material is subject toheating. Color over angle variation in the light extracted by the systemmay be minimized while maintaining high efficiency. In addition, thefraction of light converted may be made independent of the wavelengthconverting material loading and thickness of the wavelength convertingregions, such that the color point of the mixed light may be immune tominor process variations in the formation of the wavelength convertingregions.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. For example, different elements of differentembodiments may be combined to form new embodiments. Therefore, it isnot intended that the scope of the invention be limited to the specificembodiments illustrated and described.

The invention claimed is:
 1. A light-emitting device, comprising: one ormore light emitting diodes (LEDs) disposed on a surface of a substrate;a first wavelength-converting layer disposed on the surface of thesubstrate, the first wavelength-converting layer covering the entiresurface of the substrate except for portions of the surface of thesubstrate that are situated underneath any of the one or more LEDs, andthe first wavelength-converting layer having a thickness that is equalto or less than a thickness of at least one of the one or more LEDs; asecond wavelength-converting layer disposed above the one or more LEDsand the first wavelength-converting layer; and an optical cavity betweenthe first and second wavelength-converting layers.
 2. The light-emittingdevice of claim 1, wherein the thickness of the firstwavelength-converting layer is between 0.5 μm and 500 μm.
 3. Thelight-emitting device of claim 1, wherein the thickness of the firstwavelength-converting layer is between 0.5 μm and 1 mm.
 4. Thelight-emitting device of claim 1, wherein the firstwavelength-converting layer and the second wavelength-converting layerare configured to emit different colors of light.
 5. The light-emittingdevice of claim 1, wherein the first wavelength-converting layer isconfigured to transmit some of light incident on the firstwavelength-converting layer toward the substrate and transmit at least aportion of light that is reflected off the substrate back toward thesecond wavelength-converting layer.
 6. The light-emitting device ofclaim 1, wherein the first wavelength-converting layer is configured toprevent any light that is incident on the first wavelength-convertinglayer from reaching the substrate.
 7. The light-emitting device of claim1, further comprising a filter formed above the secondwavelength-converting layer that is configured to transmit a portion ofunconverted light that is emitted from the one or more LEDs whilereflecting another portion of the unconverted light towards the firstwavelength-converting layer, wherein the filter is the layer.
 8. Thelight-emitting device of claim 1, wherein the surface of the substrateis a reflective surface.
 9. The light-emitting device of claim 1,comprising a scattering layer disposed above the secondwavelength-converting layer.
 10. A light-emitting device, comprising:one or more light emitting diodes (LEDs) disposed on a surface of asubstrate; a first wavelength-converting layer disposed on the surfaceof the substrate, the first wavelength-converting layer covering theentire surface of the substrate except for portions of the surface ofthe substrate that are situated underneath any of the one or more LEDs,the first wavelength-converting layer having a thickness that is equalto or less than a thickness of any of the one or more LEDs, and thefirst wavelength-converting layer being formed of a plurality ofwavelength-converting materials disposed in different regions of thefirst wavelength-converting layer; a second wavelength-converting layerdisposed above the one or more LEDs and the first wavelength-convertinglayer; and an optical cavity between the first and secondwavelength-converting layers.
 11. The light-emitting device of claim 10,wherein the thickness of the first wavelength-converting layer isbetween 0.5 μm and 500 μm.
 12. The light-emitting device of claim 10,wherein the thickness of the first wavelength-converting layer isbetween 0.5 μm and 1 mm.
 13. The light-emitting device of claim 10,wherein each of the wavelength converting materials in the plurality isconfigured to emit a different color of light.
 14. The light-emittingdevice of claim 10, wherein the first wavelength-converting layer isconfigured to transmit some of light incident on the firstwavelength-converting layer toward the substrate and transmit at least aportion of light that is reflected off the substrate back toward thesecond wavelength-converting layer.
 15. The light-emitting device ofclaim 10, wherein the first wavelength-converting layer is configured toprevent any light that is incident on the first wavelength-convertinglayer from reaching the substrate.